+4 dB

This describes the reference used for professional level audio signals. Although it is higher than the more common line level (3.473 volts peak to peak compared to 0.895 volts), it is still not as high as the level used inside many modular synthesizers these days (10 volts ore more peak to peak). As a result, you will often need to attenuate (turn down) a modular signal plugged into a +4dB input, and turn up a +4dB audio signal fed into a modular synth – by a large amount.